Molecular biological studies have established classification of adrenergic receptors 40 years after Prof Ahlquist first classified them into alpha and beta. They are now classified into 9 subtypes consisting of 400 to 480 amino acids. They belong to a super-family of G-protein coupled receptors. Based on the amino acid sequence, the topography of adrenoceptors is depicted as a snake model twisting itself back and forth through the membrane 7 times.